The drilling of holes or bores into underground formations and particularly, the drilling of oil and gas wells, is typically accomplished using a drill bit which is attached to the lower end of an elongated drill string. The drill string is constructed from a number of sections of tubular drill pipe which are coupled at their ends to form the "drill string." The drill string extends from the drilling surface into a well or "wellbore" which is formed by the rotating drill bit. As the drill bit penetrates deeper or further into an underground formation, additional sections of drill pipe are added to the drill string.
Casing is generally installed in the wellbore from the drilling surface to various depths. The casing lines the wellbore to prevent the wall of the wellbore from caving in and to prevent seepage of fluids from the surrounding formations from entering the wellbore. The casing also provides a means for recovering the petroleum if the well is found to be productive.
A drill string is relatively flexible, being subject to lateral deflection, especially at the regions between joints or couplings. In particular, the application of weight onto the drill string or resistance from the drill bit can cause axial forces which in turn can cause lateral deflections. These deflections can result in portions of the drill string contacting the casing or wellbore. In addition, the drilling operation may be along a curved or angled path, commonly known as "directional drilling." Directional drilling also causes potential contact between portions of the drill string and the casing or well bore.
Contact between the drill string and the casing and well bore creates frictional torque and drag. In fact, a considerable amount of torque can be produced by the effects of frictional forces developed between the rotating drill pipe and the casing or the wall of the well bore. During drilling operations, additional torque is required while rotating the drill string to overcome this resistance. In addition, the drill string is subjected to increased shock and abrasion whenever the drill string comes into contact with the wall of the well bore or, where lined, the casing. Drilling tools and associated drill string devices encounter similar problems.
To alleviate these problems, drill pipe protectors are typically spaced apart along the length of the drill pipe. These drill pipe protectors were originally made from sleeves of rubber or other elastomeric material which were placed over the drill pipe to keep the drill pipe and its connections away from the walls of the casing and/or formation. Rubber or other elastomeric materials were used because of their ability to absorb shock and impart minimal wear.
Previously available drill pipe protectors have an outside diameter (O.D.) greater than that of the drill pipe joints, and were installed or clamped rigidly onto the drill pipe at a point near the joint connections of each length of drill pipe. The O.D. is specifically sized to be larger than the tool joint, but not too large as to restrict returning fluids which could result in "pistoning" of the protector in the hole. Such an installation allows the protector only to rub against the inside wall of the casing as the drill pipe rotates. Although wear protection for the casing is the paramount objective when using such drill pipe protectors, they can produce a significant increase in the rotary torque developed during drilling operations. In instances where there may be hundreds of these protectors in the wellbore at any one time, they can generate sufficient accumulative torque or drag to adversely affect drilling operations if the power required to rotate the drill pipe approaches or exceeds the supply power available.
In response to the problems of wear protection and torque build up, improvements have been directed toward producing drill pipe/casing protectors from various low friction materials in different configurations. However, such an approach again has only been marginally effective, and oil companies still are in need of an effective means to greatly reduce the wear and frictionally-developed torque normally experienced particularly when drilling deeper wells and deviated wells.
U.S. Pat. No. 5,069,297 to Krueger, et al., assigned to the assignee of the present application, and incorporated herein by reference, discloses a drill pipe/casing protector assembly which has successfully addressed the problems of providing wear protection for the casing and reduced torque build up caused by the drill pipe protectors during drilling operations. The protector sleeve in the '297 patent rotates with the drill pipe during normal operations in which there is an absence of contact between the protector sleeve and the casing, but the protector sleeve stops rotating, or rotates very slowly, while allowing the drill pipe to continue rotating within the sleeve unabated upon frictional contact between the sleeve and the casing. Thrust bearings are rigidly affixed to the drill pipe at opposite ends of the protector sleeve, and these, in combination with the internal configuration of the protector sleeve, produce a fluid bearing effect in the space between the inside of the sleeve and the outside of the drill pipe. The fluid bearing effect is produced by circulating drilling fluid through the space between the sleeve and the drill pipe so that it reduces frictional drag between the rotating drill pipe and the sleeve when the sleeve stops rotating from contact with the casing.
U.S. Pat. No. 5,803,193, to Krueger, et al., assigned to the assignee of the present application, and incorporated herein in its entirety by reference, discloses a drill pipe/casing protector assembly which provides an enhanced fluid bearing effect that reduces frictional drag between the rotating drill string and the protector sleeve during use.
Although modern drill string protector designs have improved the lubrication and protection of both the drill string and the casing, there is still a need for improved sliding lubrication. In addition, there is a need for hydraulic lift to overcome the heavy normal forces and torques encountered by the operating drill string. This problem is especially significant in extended reach drilling. In long holes and as depth increases, the friction of the drill string against the hole wall increases resulting in difficulty in putting weight on the drill bit or a tendency for the weight to surge forward then reduce in a "stickion" type process. Thus, a drill pipe protector that both reduces the torque from the drill string and increases the sliding ability of the drill string against the casing is highly desirable.